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Female parrot finches can adjust the sex of their unborn children in response to the environment where they live, according to Australian research.

The study, published in the latest Proceedings of the Royal Society B, finds that mothers exert far more control than fathers do over whether or not the couple has a son or daughter. The goal is to improve the child's survival.

"It seems likely that when there are large and predictable costs associated with producing and/or rearing either sons or daughters in a given environment, females should bias offspring sex ratios to produce the sex that will perform best in the given environment," says co-author Sarah Pryke.

"Altering offspring sex ratios in response to the quality of the local environment is likely to be highly advantageous to any species, as it should allow mothers to best match the phenotype of their offspring to the prevailing condition, and thus maximize their own fitness," adds Pryke, a researcher at the Australian National University's Research School of Biology.

Prior studies on birds, reptiles and mammals - including humans - has long suggested that this was the case, but scientists were unclear on what factors triggered the son or daughter outcome. Some researchers, for example, speculated that the overall body condition and health of the mother affected the outcome of her child's sex.

The researchers randomly assigned 56 of the female birds either a high-quality or low-quality diet. The former contained 20 per cent protein, with egg, wheat germ, a seed mixture and more, while the latter contained only 8 per cent protein.

After 12 weeks on the diet, the birds were weighed and underwent blood tests to measure various aspects of their health. Based on these tests, all of the females were in comparably good and equivalent shape both before and after the three-month study period.

However, mother birds fed the lower quality diet produced far more sons than daughters.

Sons less vulnerable

"In this case, it is adaptive for mothers to produce more sons when conditions are poor because sons are much less vulnerable to nutritional stress than daughters," says Pryke.

"For example, sons reared on poor quality diets grew faster, were healthier, fledged earlier and were much more likely to survive than daughters. Indeed, more than 51.5 per cent of daughters reared on low quality diets died before reaching parental independence compared to only 7.3 per cent of sons."

It is unclear whether or not human mothers would produce more sons or daughters when environmental conditions are poor. That will probably remain a mystery for quite a while, since, as Pryke says, "researchers can't do experimental manipulations, like in the current study" on humans.

The sex of an individual is also at least partially determined by genes, giving dads some level of indirect control over the sex outcome of their progeny.

Hormones a factor?

Yet another mystery concerns how mothers -- throughout the animal kingdom -- adjust the sex of their unborn offspring. Pryke says it's possible that hormones are involved.

Earlier research suggests that circulating levels of a potent stress hormone, corticosterone, in the female before conception or egg laying is a dominant factor.

Tim Fawcett, a research fellow at the University of Bristol's School of Biological Sciences, has studied how maternal control over the sex ratio of offspring impacts female selection of mates.

Climate change, loss of habitat and other stressors might therefore not only change sex ratios among various species, but these factors might also later subdue male sexual displays and affect female choosiness of mates.